The present invention relates generally to inkjet printing and, more particularly, to a printing technique for improving or fixing several attributes associated with inkjet printing.
As inkjet printing matured from black-only printing to include color printing (e.g., cyan, magenta, and yellow), a number of problems surfaced that had not been significant in black-only printing. Historically, the area of investigation expected to fix or improve upon many of these attributes has been black ink formulation. The following are areas typically controlled by black ink formulation and which have shown significant improvement with the implementation of this technology: media independence, area fill uniformity (mottle), dry time/throughput, finger smudge, optical density, and strikethrough (penetration of colorant to the nonprinted surface of the paper). Improvements to some of these attributes have been addressed through either printer hardware or writing systems development efforts. Some of the prior solutions are discussed below.
Dry time/throughput. Solutions here include decreasing the drop volume and inclusion of a heater or fuser into the printer. Decreasing the drop volume can significantly decrease dry times, but can lead to other architectural and logistical problems. Heaters or fusers can perform a similar function, but can be unattractive due to requirements of space, power consumption, and cost.
PQ (Print Quality). Management of drop placement, shape and variability are typical architectural and pen characteristics used to improve PQ. Improvements in these areas are extremely important, but are also very time consuming and costly. In addition, there is a limit to how much improvement may be achieved. Regardless, the benefits of this new technology are in addition to these solutions and are additive to the solutions already being utilized.
Masking missing or misdirected nozzles and banding. Multipass printing and various shingle masks have both been implemented to reduce the appearance of banding as well as missing, weak, or misdirected nozzles. The effectiveness of these techniques is limited by the ability to implement the correct shingle mask for all occasions as well as limiting within-printhead nozzle variations. These problems are inherently difficult to address due to the high contrast nature of misplaced black drops on a typically white background. Application of color ink under and over the black printed area reduces the background from white to a secondary color, significantly lowering contrast and rendering many defects much less obvious to the customer. In addition, the previous solutions should work in combination with the new under/over printing technology discussed herein.
Black ink reliability. Improvements to reliability have typically been left to ink development and further enhanced by complicated servicing algorithms. Limitations of this approach include constraining the black ink formulation space and the necessity of a highly functional and complex service station adding additional cost and limiting throughput.
Most solutions proposed for improving one or more of the foregoing attributes often result in degradation of other attributes. Accordingly, efforts continue to develop solutions to improving most, if not all, of the foregoing attributes without degradation of other attributes.
In accordance with the present invention, one or more reactive color inks are printed under and/or over black text and graphics areas. That is to say, a text or graphics area is first defined. Then, three different inks are applied to this area in various amounts in the following order: color, black, color. The use of under-and/or over-printing of black text and/or graphics by reactive color inks improves many important characteristics of inkjet printing. Specifically, improvements include, but are not limited to the following:
(1) Drytime: Printing the color ink underneath the black ink allows for faster black ink penetration into the substrate, thereby reducing the time to obtain dry areas of black ink. As a result of the faster dry time, throughput can be increased significantly without compromising finger smudge or blotting performance.
(2) Smudgefastness: This characteristic is related to the drytime of the ink. This is in reference to the amount of ink that is transferred when a user touches the printed area shortly after printing. An ink with a long drytime requires that the user wait for the ink vehicle to either absorb into the substrate or evaporate before handling the printout. Under/over printing shortens the drytime of the ink which allows a user to manipulate the printed pages sooner with a lower chance of smudging the printed page.
(3) Media Independence/Image Quality: Under/over printing can also enhance the quality of the printed image and black optical density. One disadvantage of inkjet printing is in that it can be very media sensitive. Certain papers react differently to inks due to variations in density, materials, fillers, and physical properties of the paper. These characteristics affect the rate of ink penetration and may be highly varied between paper types and even within a single sheet of paper. Some papers have a slow absorption rate, and the ink vehicle tends to remain on the paper surface until most of it has evaporated. Some papers cause the ink to penetrate unevenly into the page, resulting in a black area fill which appears nonuniform. Other papers tend to absorb ink quickly, leaving less for surface evaporation and resulting in low optical density. Under/over printing allows a more uniform and higher optical density area coverage across a wider paper set. Typically pigmented black inks containing relatively low surfactant levels and low organic contents have difficulty xe2x80x9cwettingxe2x80x9d paper surfaces. While these formulations may be necessary to obtain superior print quality, this can result in both slower dry times and nonuniform performance over even relatively small areas. The color ink used to underprint the black ink xe2x80x9cwetsxe2x80x9d the page more effectively, allowing the black ink vehicle to absorb into the paper more quickly. At the same time, reactants in the color ink cause the black pigment particles to quickly destabilize or precipitate on the surface of the page. By keeping a higher percentage of the pigment particles on the surface of the paper, the black image is more uniform, more consistent, and has a higher optical density. At the same time, prewetting the paper surface allows for a faster dry time with all of the related benefits.
(4) Strikethrough: On certain media, black pigment penetrating the paper can be seen on the opposite side of the printed image. Small, concentrated, black points can be seen as a result of black ink penetrating through pores in the paper. Underprinting the black areas increases the rate of penetration of the black ink vehicle and increases the total amount of ink vehicle present, but precipitates a higher percentage of the pigment particles on the printed surface of the page. This reduces and in many cases eliminates the amount of small, black points that can be seen from the nonprinted side of the page.
(5) No dedicated pens required: Under/over printing can be accomplished with pen and ink sets currently available. In a proposed new inkjet printer, the pen order will be CKMY (cyan, black, magenta, yellow). This allows under/over printing on each pass of a bidirectional printmode. During any printed swath, one color can underprint while another overprints the black ink. In the reverse direction, the same color inks may be used, but with reversed roles. The amount of ink that is under/overprinted can be optimized to help control bidirectional hue shift caused by underprinting with the same color inks, but in a different order depending upon print direction. Specifically, black may be underprinted by cyan (or magenta) and overprinted by magenta (or cyan); placement of the black pen between cyan and magenta facilitates this printing scheme.
(6) Black Text and Graphics Quality: Text and graphics quality is improved simply by the increase in optical density. For text this effect is dramatic on some papers and negligible on others. For graphics, the effect can be even greater and for a larger variety of papers.
(7) Waterfastness: The mechanism by which shorter drytime is achieved with underprinting also improves immediate waterfastness. The black vehicle penetrates more quickly and the pigment precipitates on the surface of the paper. Without this action more vehicle would be on the surface of the paper with pigment still stabilized and more mobile in solution. Since the vehicle has not had a chance to fully absorb, exposing the printed area to water could easily wash away the vehicle and pigment. Noticeable improvements in waterfastness from 0 to 5 minutes can be obtained. This improvement is dependent upon the choice of both ink and paper.
(8) Missing/Misdirected Nozzles: The ability to reduce visibility of print defects such as missing, weak, and/or misdirected nozzles is due to the decrease in contrast between the background of the printed area and the printed area itself. Because color ink is applied to the paper both under and over the black printed area, if a drop or small section of black drops is missing, misdirected, or smaller than expected, the background in that area makes it more difficult to observe. Due to the background in the black printed area now being a composite of two color inks a situation of much lower contrast between the printed area and the defect area exists. Due to the lower contrast, the defective area is significantly more difficult to observe.